The present invention relates to methods for reducing the amount of chromium present in non-volatile residue (NVR). More specifically, the invention relates to methods for reducing the amount of chromium present in non-volatile residue obtained during the production of cyclohexanol and cyclohexanone by air oxidation of cyclohexane.
Non-volatile residue (xe2x80x9cNVRxe2x80x9d), obtained during the production of cyclohexanol and cyclohexanone by air oxidation of cyclohexane, is an aqueous mixture of organic and inorganic compounds comprising butyric acid, valeric acid, caproic acid, 6-hydroxy-caproic acid, glutaric acid, succinic acid, adipic acid, oligomers of one or more of these organic compounds, organic esters of phosphoric acid, including diethylhexyl phosphoric acid (DEHPA), chromium compounds, including chromium complexes involving the above compounds, and water.
NVR of the above type is produced as waste stream in cyclohexane oxidation processes and is generally disposed of as boiler fuel. Combustion of NVR in a boiler might cause emission of chromium with boiler stack gases. Expensive stack gas treatment is required in order to maintain an acceptable low concentration of chromium in the stack gas. Alternative routes of disposal of NVR are also expensive.
It is, therefore, desirable to have a method to significantly reduce the amount of chromium in NVR so that, upon combustion, it will produce a stack gas with an acceptable low concentration of chromium. It is also desirable to have a method for reducing chromium content of NVR in a way that its organic content is not substantially reduced. In other words, a new waste organic content stream should not be created.
Techniques available in the art for reducing the concentration of chromium in aqueous waste products involve reduction of hexavalent chromium to trivalent chromium using a suitable reducing agent and precipitation of trivalent chromium using a suitable precipitant. A two-step process is described in U.S. Pat. No. 5,308,501 for treating heavy metal solutions with ferrous sulfate and caustic to precipitate chromium as chromium hydroxide. In JP 57075186 chromium and fluorine is removed from waste water by reducing the hexavalent chromium to trivalent chromium with ferrous sulfate, sodium sulfite or sodium bisulfite and adding calcium hydroxide until chromium is precipitated.
It is well known in the art that phosphoric acid, DEHPA and organic dicarboxylic acids form strong complexes with chromium. No method is available for precipitating chromium in the presence of complexing agents, such as organic dicarboxylic acids and DEHPA.
WO 9914179 describes a method for recovering catalyst from a mixture of catalyst and water obtained from hydrocarbon oxidation. The method used reduction of the temperature, or distilling off amounts of water, or both. For NVR, cooling or evaporating portion of water cause solidification of some of the organic compounds.
U.S. Pat. No. 4,720,592 teaches that catalysts and additives that may be used during the cyclohexane oxidation process may also end up in the NVR.
US 2002042722 teaches in-situ treatment of an underground area containing hexavalent chromium and other metals. The in-situ treatment and remediation of an underground area comprises the use of ferrous sulfate and acid such as sulfuric acid or phosphoric acid, as the first reactive solution to de-complex chromium and initiate reduction of hexavalent chromium to trivalent chromium. A second reactive solution comprising of hydrogen peroxide and acid destroys organic ligands.
DE 4336225 describes a method for toxic chromate removal from heat-storage bricks. The method uses ferrous sulfate in sulfuric acid to reduce the hexavalent chromium to trivalent chromium. The method of the present invention does not require the reduction of hexavalent chromium to trivalent chromium.
U.S. Pat. No. 5,308,501 describes a two-step process for treating heavy metal solutions with ferrous sulfate and caustic to precipitate chromium as chromium hydroxide. The process does not decompose the chromium ligands.
There are many shortcomings to the methods that are described in the art. When the chromium catalyst level is in the tenths to hundredths ppm range, the chromium will not precipitate at low temperatures. If the water is distilled off, the organic components in NVR will precipitate rather than the desired chromium. Because of the complex composition and physical properties of NVR none of the above methods work to reduce or substantially eliminate chromium in NVR.
The object of the present invention is to provide a method for reducing or substantially eliminate the amount of chromium, while eliminating the step of reducing hexavalent chromium to trivalent chromium.